Now you need center signal from RC. Connect RX to PCB and Arduino connect with PC. Control stick on TX set to neutral. Serial monitor write some values. Copy this values to Excel and make average value ( about 50 values is OK). Repeat to second channel.

// calibration RX signal
Serial.println( value_ch1);

and

// calibration RX signal
Serial.println( value_ch2);

Servo modifying

Servo with metal gear use to wheel. You must modifying to countinous rotating. Cut of potentiometer, add 2 resitors, cut of pin on tooth wheel.